1. Field of the Invention
The present invention relates to methods for making an electron emission device and an image forming apparatus, and to an apparatus for making the same.
2. Description of the Related Art
Conventional electron emission devices are classified into thermal electron source devices and cold cathode electron source devices. The cold cathode electron source devices include field emission (hereinafter referred to as FE) types, metal-insulator-metal (hereinafter referred to as MIM) types, and surface conduction types.
FE type devices are disclosed by, for example, W. P. Dyke & W. W. Dolan ("Field Emission", Advances in Electron Physics, Vol. 8, 89 (1956)), and by C. A. Spindt ("Physical Properties of Thin-Film Field Emission Cathodes with Molybdenum Cones", J. Appl. Phys., Vol. 47, 5248 (1976)). MIM type devices are disclosed by, for example, C. A. Mead % ("The Tunnel-Emission Amplifier", J. Appl. Phys., Vol. 32, 646 (1961)). Surface conduction type devices are disclosed by, for example, M. I. Elinson (Radio Eng. Electron Phys., Vol. 10, 1290 (1965)).
In surface conduction electron emission devices, when a current flows along the plane of a thin film with a small area formed on a substrate, electrons are emitted. Examples of thin films disclosed as surface conduction electron emission devices include an SnO.sub.2 thin film by Elinson as described above, a gold thin film by G. Dittmer (Thin Solid Films, Vol. 9, 317-328 (1972)), an In.sub.2 O.sub.3 /SnO.sub.2 thin film by M. Hartwell and C. G. Fonstad (IEEE Trans. ED Conf., p. 519 (1975)), and a carbon thin film by H. Araki et al. (Sinku (Vacuum), Vol. 26, No. 1, 22 (1983)).
FIG. 25 shows a configuration of the above device by M. Hartwell as a typical example of a surface conduction electron emission device. A conductive film 4 having an H shape is formed on a substrate 1. The conductive film 4 is composed of the above-described composite metal oxide. The conductive film 4 is subjected to an electrifying process generally called "electrifying forming" to form an electron emitting section 5. In the drawing, two device electrodes have a total length L in the range of 0.5 to 1.0 mm, and a width W' of approximately 0.1 mm.
In the surface conduction electron emission device, the electron emitting section 5 is generally formed by the "electrifying forming" process of the conductive film 4 prior to electron emission. In the electrifying forming, a voltage is applied to two ends of the conductive film 4 to locally destruct, deform or modify the conductive film 4. As a result, the electron emitting section 5 having high electrical resistance is formed. The electron emitting section 5 includes cracks and electrons are emitted near the cracks.
Examples of arrays of many surface conduction electron emission devices are ladder-type electron sources disclosed in, for example, Japanese Patent Application Laid-Open Nos. 64-31332, 1-283749, and 2-257552, in which many lines of surface conduction electron emission devices are arranged, and two ends (electrodes) of each devices are connected to lead lines (common lead lines).
An array of surface conduction electron emission devices enables production of a planar display device similar to a liquid crystal display device. U.S. Pat. No. 5,066,883 discloses such a display device which comprises a combination of an electron source including many surface conduction electron emission devices and a fluorescent coating which is irradiated with electrons from the electron source to emit visible light.
Preferably, a voltage is applied to the electron emission device subjected to electrifying forming in an atmosphere containing an organic substance in order to improve electron emission characteristics (hereinafter referred to as an activation step). The voltage applied in the activation step is substantially equal to the voltage applied in the forming step. Carbon and/or carbonaceous materials are deposited on and near the electron emitting section 5 during the activation step, as disclosed, for example, in European Patent Application Laid-Open No. 0660357.